Mishkat al-Masabih 4 Prayer كتاب الصلاة 747

وَعَنِ ابْنِ عُمَرَ قَالَ: كَانَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يُصَلِّي فِي السَّفَرِ عَلَى رَاحِلَتِهِ حَيْثُ تَوَجَّهَتْ بِهِ يُومِئُ إِيمَاءً صَلَاةَ اللَّيْلِ إِلَّا الْفَرَائِضَ وَيُوتِرُ على رَاحِلَته

Ibn ‘Umar said that on a journey God’s Messenger would pray on his riding beast in whatever direction it turned, making a sign with his head* in prayer during the night, but not in the obligatory prayers; and he would observe a witr on his riding beast. *To indicate the bowing and the prostration. (Bukhari and Muslim.)